The present invention relates to a liquid crystal display device and a driving method for the same, and more particularly, relates to a liquid crystal display device suitably used for display of moving images and a driving method for the same.
In recent years, liquid crystal display devices (LCDs) have increasingly come into widespread use. Among various types of LCDs, mainstream has been a TN LCD in which a nematic liquid crystal material having positive dielectric anisotropy is twisted. The TN LCD however has a problem of being large in visual angle dependence that results from the alignment of liquid crystal molecules.
To improve the visual angle dependence, alignment-divided vertical alignment LCDs have been developed, and use of these LCDs is expanding. For example, Japanese Patent Gazette No. 2947350 (Literature 1) discloses an MVA LCD as one of the alignment-divided vertical alignment LCDs. The MVA LCD, which includes a vertical alignment liquid crystal layer placed between a pair of electrodes to present display in the normally black (NB) mode, is provided with domain regulating means (for example, slits and/or protrusions) to enable liquid crystal molecules in each pixel to fall (tilt) in a plurality of different directions during application of a voltage.
Recently, needs for displaying moving image information have rapidly increased, not only in LCD TVs, but also in PC monitors and portable terminal equipment (such as mobile phones and PDAs). To display moving images with high definition on LCDs, it is necessary to shorten the response time (increase the response speed) of the liquid crystal layer, so that a predetermined grayscale level can be reached within one vertical scanning period (typically, one frame).
As for the MVA LCD, Literature 1 mentioned above, for example, discloses that the black-to-white response time can be shortened to 10 msec or less. Literature 1 also describes that regions different in the distance between protrusions are provided in each pixel to give regions different in response speed, to thereby attain improvement in apparent response speed without reducing the aperture ratio (see FIGS. 107 to 110 of Literature 1, for example).
As a driving method that can improve the response characteristic of LCDs, known is a method in which a voltage higher than a voltage (grayscale voltage) corresponding to the grayscale level to be displayed (this voltage is called an “overshoot (OS) voltage”) is applied (this method is called “overshoot (OS) driving”). With application of an OS voltage, the response characteristic in grayscale display can be improved. For example, Japanese Laid-Open Patent Publication No. 2000-231091 (Literature 2) discloses an MVA LCD adopting the OS driving. Literature 2 however describes that an OS voltage should not be applied when a shift is made from the black display state to a high-luminance grayscale display state (see FIG. 8 of Literature 2). The reason is described as that the transmittance will be overshot if an OS voltage (e.g., voltage 1.25 times as high as that for giving the target transmittance) is applied at a shift from the black display state to a high-luminance grayscale display state as is applied at a shift from the black display state to a low-luminance grayscale display state or to the white display state.
However, as a result of examinations done by the inventors of the present invention, it has been found that a new problem arises when the OS driving is adopted for alignment-divided vertical alignment LCDs such as the MVA LCD described above. This problem will be described with reference to FIGS. 20A and 20B.
FIGS. 20A and 20B are views diagrammatically showing the state of display observed when a square 92 of a certain grayscale level (for example, level 32/255) is moved in a black background 90 (for example, level 0), in the cases of driving a conventional MVA LCD by a normal driving method (FIG. 20A) and driving the same by the OS driving (FIG. 20B). Note that the “level 32/255” is a grayscale level giving a luminance of (32/255)2.2 with respect to the luminance in black display (during application of V0) as 0 and the luminance in white display (during application of V255) as 1 when the grayscale display is set as γ2.2. The grayscale voltage giving this luminance is denoted by V32.
When no OS driving is adopted, the response speed of the alignment-divided vertical alignment LCD is low. Therefore, as diagrammatically shown in FIG. 20A, a left edge 92a of the square 92 located downstream of the moving direction may not be observed clearly in some cases. When the OS driving is adopted, the response speed is improved, and thus as diagrammatically shown in FIG. 20B, the edge 92a is observed clearly. However, a new phenomenon arises in some cases, in which a dark belt 92b is observed at a position of the square a little apart from the edge 92a. 
The present inventors have examined the cause of the above problem in various ways, and found that the above phenomenon is a new problem that has never occurred as long as the OS driving is adopted for conventional TN LCDs, and results from the alignment division done with the alignment regulating means (domain regulating means) placed linearly (in a stripe shape) in each pixel in alignment-divided vertical alignment LCDS.
In view of the above, a main object of the present invention is providing an alignment-divided vertical alignment LCD permitting high-definition moving image display.